In general, holographic recording for recording information in a recording medium through the use of holography is performed by superimposing light that carries image information on reference light within the recording medium and by writing a resultingly generated interference pattern into the recording medium. For reproducing the information recorded, the recording medium is irradiated with reference light such that the image information is reproduced through diffraction derived from the interference pattern.
In recent years, volume holography, or digital volume holography in particular, has been developed and is attracting attention in practical fields for ultra-high density optical recording. Volume holography is a method for writing a three-dimensional interference pattern by making positive use of a recording medium in a direction of its thickness as well, and is characterized in that it is possible to enhance the diffraction efficiency by increasing the thickness of the medium, and a greater recording capacity can be achieved by employing multiplex recording. Digital volume holography is a computer-oriented holographic recording method which uses the same recording medium and recording method as with the volume holography, whereas the image information to be recorded is limited to binary digital patterns. In the digital volume holography, analog image information such as a picture is once digitized and developed into two-dimensional digital pattern information, and then it is recorded as image information. For reproduction, this digital pattern information is read and decoded to restore the original image information for display. Consequently, even if the signal-to-noise ratio (hereinafter referred to as SN ratio) during reproduction is somewhat poor, it is possible reproduce the original information with extremely high fidelity by performing differential detection and/or error correction on the binary data encoded.
By the way, typical recording apparatuses that record information on a disk-shaped recording medium through the use of light comprise an optical head for irradiating the recording medium with light for information recording. In these recording apparatuses, the recording medium is rotated while the optical head irradiates the recording medium with the light for information recording, thereby recording information on the recording medium. In these recording apparatuses, a semiconductor laser is typically used as the light source for generating the light for information recording.
For holographic recording, as in the typical recording apparatuses described above, the recording medium can also be rotated while the recording medium is irradiated with information light and reference light so that information is recorded in a plurality of information recording areas of the recording medium in succession. In this case, as with the typical recording apparatuses, a practical semiconductor laser is desirably used as the light source of the information light and the reference light.
When existing photosensitive material for holography is used to make a recording medium for holographic recording and this recording medium is rotated while the recording medium is irradiated with the information light and the reference light that are generated by a semiconductor laser, however, there arises a problem as follows. That is, in this case, it is difficult to give exposure energy sufficient to record information in the form of an interference pattern to a single information recording area of the recording medium in a short time. To cope with this, the exposure time may be extended to give sufficient exposure energy to a single information recording area. Nevertheless, this can increase the moving distance of the information recording areas within the exposure time for a single information recording area, resulting in deterioration in information accuracy.
Here, the forgoing problem will be detailed with a concrete example. If a high-power light source such as a pulse laser is used as the light source instead of a semiconductor laser, then it is satisfactorily possible to record information on the recording medium while rotating the recording medium. Take, for example, the case where a pulse laser which has a maximum power of several kilowatts and is capable of generating pulsed light of several tens of nanoseconds is used as the light source. Here, assume the light intensity on the recording medium to be 200 W. The pulse width of the pulsed light shall be 20 ns, and the linear speed of the information recording areas 2 m/s. In this case, the moving distance of the information recording areas within the exposure time for a single information recording area is 0.04 μm, not exceeding one tenth of the wavelength of the light, so that it is possible to maintain sufficient information accuracy. The use of such a pulse laser as described above for the light source is impractical, however.
Next, turn to the case where a semiconductor laser is used as the light source. Here, assume that the light intensity on the recording medium is 20 mW and the linear speed of the information recording areas is 2 m/s. In this case, an exposure time of 200 μs, or 10000 times as much as with the foregoing pulse laser, is required in order to give a single information recording area the same amount of exposure energy as with the pulse laser. The moving distance of the information recording areas for this exposure time reaches 400 μm, making it difficult to record information in the form of interference patterns.